There is a universal problem associated with rechargeable battery systems known as the "key chain" problem. It is known as the "key chain" problem because when a battery with exposed terminals is placed in a pocket of a user, any conductive object, such as a key chain for example, that comes in contact with the terminals can provide a short circuit. Such a short circuit can produce large currents that may cause the key chain or other conductive object to increase in temperature.
To prevent such a phenomenon from occurring, prior art battery systems have typically included a series element, such as a fuse or positive temperature coefficient (PTC) resistor, coupled between one external terminal and the cell. When large currents flow, either the fuse clears or the PTC becomes an open, thereby stopping the flow of current.
More modern systems employ transistors coupled in series between a terminal and the cell. FIG. 1 shows such a prior art system. In the prior art system of FIG. 1, it may be necessary to protect the battery cell from excessive current flow in either direction. For example, a "key-chain" short between terminals 108 and 109 may cause too much current flow in the cell discharge direction, with excessive current flowing from cell terminal 101 to the external terminal 108. Also, a cell may be overcharged from too much current flow in the cell charge direction, with excessive current flowing from the external terminal 108 to the cell terminal 101. To protect against excessive current flow in both directions two field effect transistors (FETs) are used due to the parasitic diode component associated with each. When power semiconductor FETs are manufactured, the body substrate, coupled with the method of manufacture create a parasitic diode. Referring to FIG. 1, diode 102 is a parasitic diode component of transistor 103.
This parasitic diode causes problems in battery circuit designs. While transistor 103 can prevent current from flowing from the cell 101 to the positive terminal 108, the transistor 103 cannot prevent current from flowing in the opposite direction due to the diode. In other words, if transistor 103 is open, and diode 102 becomes forward biased, current will flow through the diode 102.
To prevent bidirectional current flow, designers must use another transistor 106 that has its parasitic diode 107 aligned in the opposite direction. In such a fashion the charging transistor 106 opens to prevent current from flowing from the terminal 108 to the cell 101, and the discharge transistor 103 opens to prevent current from flowing from the cell 101 to the terminal 108. In each case, the parasitic diode is reverse biased preventing current flow.
In addition, as known in the prior art, transistor 106 may be used to regulate current flow in the charge direction. In this case, the FET 106 is used in its linear ohmic operating mode to adjust the desired charging current. A charge regulator system 105 may be used to accomplish this operation, as is common in the art. Likewise, transistor 102 may be used to regulate current flow in the discharge direction, with the FET 102 used in its linear ohmic operating mode to adjust the desired discharge current. A discharge regulator system 104 may be used to accomplish this operation, as is common in the art. The linear ohmic operating mode is understood herein to include also the states where the transistor being controlled may be completely "on" (saturated), or completely "off" (cutoff), or the transistor may be in any ohmic conduction region between these extremes. Because of the prior art application and usage of these two transistors, transistor 106 may be typically referred to as the "Charge FET," and transistor 102 may be typically referred to as the "Safety FET."
The use of two transistors to regulate and control cell current flow in either direction from a battery cell is an expensive and bulky solution to the problem of cell safety protection. There is a need for an improved battery circuit to provide a charging system that reduces the number of components and complexity of the circuit, and still provides desirable safety features.